BARRIER SYSTEMS ON MSE WALLS

Researchers have some new insight on what happens to safety barrier systems on mechanically stabilized earth walls during a crash.

MSE walls are familiar structural elements on Utah's highways. How barriers on MSE walls perform in a crash is the subject of a recent study.

According to the Texas Transportation Institute, approximately10 million square feet of MSE wall is constructed every year in the United States. UDOT has a database of over 700 walls in use to support backfill around bridges and freeway ramps or for retaining walls along freeway corridors. MSE walls are relatively new features in the transportation world; UDOT’s oldest walls were built about 30 years ago. Useful life of the walls is expected to be 75 years.

Test MSE wall under construction

Because of the newness of MSE walls, research is needed in many aspects of design and construction. UDOT Geotechnical Engineer Grant Gummow recently participated in a TRB sponsored National Cooperative Highway Research Program study that focuses on concrete roadside barrier systems on MSE walls – that is, how the barrier system performs as a safety device and how the system can be designed to protect the MSE wall from damage.

MSE walls are built in lifts (layers) of fill with steel or geosynthetic reinforcement. Concrete panels or blocks are used as facing to retain the reinforced backfill. Facing materials are anchored using welded galvanized steel wire mesh, steel straps, or geosynthetic grids embedded in the retained backfill.

MSE walls barrier system design was borrowed from bridge design principles. Barriers are usually attached to a coping, which in turn is attached to a moment slab (footing) covered with overburden soil. The components of the system work together to avert a vehicle from tumbling over a wall, redirect the vehicle in a path that is parallel to traffic and to distribute the load over a wide area to limit potential damage to the internal structure of the MSE wall.

UDOT’s barrier systems, and across the country, seem to be working well. Without objective studies, “it’s difficult to know how conservative the design is,” says Gummow. “We don’t want to over-design.” Safety is paramount, but a design that’s too conservative wastes funding and resources.

Crash testing

Researchers tested a variety of MSE wall reinforcements and two barrier types, Jersey and a vertical barrier, all representing current building and design practice. The MSE walls were built for the study and outfitted with strain gauges to measure the degree of deformation that occurs when pressure is applied.

Crash testing took place in stages. First, a jack was used to push against the barrier. Next testers used a bogie vehicle – a steel frame with weights to approximate a car – to crash into the barrier. The third type of testing was done with an actual car. The unmanned crash car was pulled by another vehicle using a cable and pulley system that released the crash car at the desired angle at a speed of approximately 63 mph.

Computer modeling

A Computer model was used to conduct a Finite Element Analysis. The model used a very detailed computer image of a standard sized pickup truck that showed, right down to minute bumper movements, what happens to a truck in an actual crash. The barrier in the model was set up to show how the components of the system, including steel reinforcement, are impacted in a crash. By using computer modeling, the researchers were able to vary angles and speeds and then measure and assess damage to the truck, barrier system and MSE wall.

The results of the study provide the basis for new Load-and-Resistance Factor Design specifications for barrier systems on MSE walls.